Foundry test: How to Write Basic Tests

src/Counter.sol

@@ -1,14 +1,22 @@
 // SPDX-License-Identifier: UNLICENSED
-pragma solidity ^0.8.13;
+pragma solidity ^0.8.20;
 
 contract Counter {
-    uint256 public number;
+    uint public count;
 
-    function setNumber(uint256 newNumber) public {
-        number = newNumber;
+    // Funcion to get the current count
+    function get() public view returns (uint) {
+        return count;
     }
 
-    function increment() public {
-        number++;
+    // Function to increment count by 1
+    function inc() public {
+        count += 1;
+    }
+
+    // Function to decrement count by 1
+    function dec() public {
+        // thid function will fail if count = 0
+        count -= 1;
     }
 }

test/Counter.t.sol

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: UNLICENSED
 pragma solidity ^0.8.13;
 
-import {Test, console2} from "forge-std/Test.sol";
+import {Test, stdError} from "forge-std/Test.sol";
 import {Counter} from "../src/Counter.sol";
 
 contract CounterTest is Test {
@@ -9,16 +9,26 @@ contract CounterTest is Test {
 
     function setUp() public {
         counter = new Counter();
-        counter.setNumber(0);
     }
 
-    function testIncrement() public {
-        counter.increment();
-        assertEq(counter.number(), 1);
+    function testInc() public {
+        counter.inc();
+        assertEq(counter.count(), 1);
     }
 
-    function testSetNumber(uint256 x) public {
-        counter.setNumber(x);
-        assertEq(counter.number(), x);
+    function testFailDec() public {
+        counter.dec();
+    }
+    
+    function testDecUnderFlow() public {
+        vm.expectRevert(stdError.arithmeticError);
+        counter.dec();
+    }
+
+    function testDec() public {
+        counter.inc();
+        counter.inc();
+        counter.dec();
+        assertEq(counter.count(), 1);
     }
 }